As disclosed in U.S. patent applications Ser. No. 776,831, filed Sept. 17, 1985 and Ser. No. 866,216, filed May 22, 1986, the disclosures of which are incorporated by reference, liquid crystal display devices which exploit the light scattering and transmitting properties of discrete quantities of liquid crystal can be fabricated by polymerization induced phase separation of homogeneous solutions of liquid crystal and various synthetic polymers. The liquid crystal-polymeric material can be formed as a film or sheet and placed between transparent conducting electrodes to form an electrically switchable light shutter.
Liquid crystals useful for light scattering displays have two indices of refraction: an extraordinary index of refraction n.sub.e measured along the long axis of the liquid crystals, and a smaller ordinary index of refraction n.sub.o, measured in a plane perpendicular to the long axis. The long axis of the liquid crystal defines its optic axis.
Entrapping liquid crystals in microdroplets in a polymeric matrix having an index of refraction n.sub.s will result in a device which either scatters or transmits incident light depending upon the relationship among the indices of refraction and dependent upon the microdroplets being of a size to scatter incident light, e.g., on the order of 0.1 to 10 microns.
In the absence of an applied field, the optic axes of the microdroplets have no preferred direction in which to point, so that incident light encounters a mismatch between the n.sub.s of the matrix and the n.sub.e of the microdroplets. The result of the mismatch is that the light is scattered and the device appears opaque. Application of an electric field across the sheet of liquid crystalline-polymeric material causes the optic axes to align parallel to the field and normal to the surface of the sheet. Incident light detects no mismatch between n.sub.o and n.sub.s and is transmitted so that the device appears clear.
Liquid crystal devices as disclosed in U.S. patent application Ser. No. 776,831 do not have an image memory. Images formed by contrasting opaque and clear areas are displayed on these devices by the constant application of a field to those areas desired to appear clear. Once the field is removed, the clear areas switch back to opaque in about 10-100 seconds.
U.S. patent application Ser. No. 866,216 teaches liquid crystal-polymeric materials having optical memory in that light scattering devices that include the materials can be manipulated to display contrasting opaque and clear areas which are field independent. The memory materials of U.S. patent application Ser. No. 866,216 are, in general, made by forming a homogeneous solution of liquid crystal and thermoplastic polymer heated to a temperature effective to dissolve the liquid crystal, and then cooling the solution to affect phase separation of the liquid crystal and the formation of microdroplets. Memory is made possible by the selection of a liquid crystal which remains in the liquid crystalline state at temperatures above the softening temperature of the thermoplastic. Since the liquid crystal is in the liquid crystalline state while the thermoplastic is soft, the optic axes of the microdroplets can be aligned by an external field. Maintaining the external field while the thermoplastic is rehardened results in a material in which the microdroplets exposed to the field remain aligned upon removal of the field. An external field applied to the entire surface of a film during hardening of the thermoplastic will result in a wholly transparent material; a patterned field as, for instance, an alphanumeric character, applied during hardening of the thermoplastic will result in a material displaying a clear (transmitting) character in an opaque (scattering) field. The clear area or areas will remain so until the random alignment of the optic axes is restored by reheating the material and cooling it in the absence of a field.
While this type of memory achieved by the selective application of thermal and electric or magnetic fields has many useful applications, it would be advantageous in many instances to have a memory not dependent upon thermal energy and characterized by faster switching times and greater transparencies than previously achievable.